A number of manufactured articles require coatings over at least a portion thereof, such as primer coatings, protective coatings such as a clear or hard coatings, antimicrobial coatings, and the like. In particular, medical devices, such as, for example, catheters and guide wires, often contain specialty coatings, including hydrophilic lubricious coatings, antimicrobial coatings, and the like for optimal performance. The device or other article is coated such as by spray coating, spin coating, curtain coating, or dip coating, and is subsequently cured.
One type of coating includes a thermally curable solvent-based coating. Convection is used to drive off the solvent and cure the coating. Thermally curable coatings, however, often have long curing cycles, sometimes up to twelve hours or more, for a satisfactory or complete cure. To maximize output of coated articles, long tunnel ovens are built so that multiple articles can be moved through the tunnels at once. However, these tunnels can stretch for a hundred feet or more, in order for an article to completely cure along its path through the tunnel, or can require multiple passes to achieve cure. These tunnel ovens can be expensive, inefficient, and can result in inconsistent curing if the air flow and temperature is not closely controlled.
Infrared (IR) curable coatings are also known in the art. Infrared energy is a form of radiation, which falls between visible light and microwaves in the electromagnetic spectrum. Like other forms of electromagnetic energy, IR travels in waves and there is a known relationship between the wavelength, frequency and energy level. That is, the energy (temperature) increases as the wavelength decreases.
Unlike convection, which first heats air to transmit energy to the part, IR energy may be absorbed directly by the coating. It may also be reflected or transmitted to the substrate. IR curing results in significantly shorter cycle times than thermal cure because of its intensity. The heat generated from the IR oven also drives off any solvent and aids in the ultimate cure. However, for sufficient curing to be attained, IR curing systems can generate large amounts of heat during the cure cycle causing delicate or heat-sensitive articles, such as thermoplastic catheters, to deform or melt, and/or the coating to degrade or become over-cured such that it is brittle.
UV-curable coatings are used often for coating applications because they tend to require shorter cycle times with less heat. Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm. UV-A, or long wave UV, has a range of wavelengths from about 315 nm to about 400 nm. UV-B, or medium wave UV, has a range of wavelengths from about 315 nm to about 280 nm. UV-C, or short wave UV, has a range of wavelengths from about 280 nm to about 100 nm.
A UV-curable coating or resin system typically includes a photoinitiator, monomers and/or oligomers, and other components as needed. The photoinitiator, upon exposure to the UV light, decomposes to produce an abundance of free radicals. These free radicals then cause monomers or oligomers present in the coating to “open up” and combine with other monomers or oligomers to form polymers, thereby cross-linking or curing the coating. Different photointiators absorb UV light most efficiently at different wavelengths. Therefore, the UV resin or coating systems are specifically tailored to the type of lamp used, or vice versa.
UV curing is fast, and can typically be accomplished in 600 seconds or less, which permits UV ovens to be confined and compact, allowing for faster production rates than other cure methods, such as thermal cure, that require substantial oven dwell times. The quick cure also minimizes substrate heating, which is a great advantage when curing films on heat-sensitive thermoplastic substrates, such as catheters.
Cure by UV is accomplished in shielded and enclosed chambers saturated with high intensity electrically generated UV light. For total curing to take place in a UV-curable coating system, the UV light must activate as many of the photoinitiator molecules as possible, which means that the light must be exposed to all of the coating areas to be cured, and therefore, the UV light must be kept close to the part or article being cured. In UV-curable systems, the energy of the UV light decreases quickly, i.e. it decreases as a square of the distance, which quickly affects the cure of the coating. Because of this requirement, the currently available systems and methods for UV-curing of coatings suffer from the ability to be massively scaled-up to increase product throughput without compromising coating and/or product quality.
High intensity UV lamps, such as arc lamps, have been used to shorten cycle times in order to increase throughput. For example, xenon-mercury short-arc lamps have been used. In these lamps, the majority of the light is generated in a tiny, pinpoint sized cloud of plasma situated at the tip of each electrode. The light generation volume is shaped like two intersecting cones, and the luminous intensity falls off exponentially moving towards the center of the lamp. Xenon-mercury short-arc lamps have a bluish-white spectrum and extremely high UV output. Furthermore, the output of the arc lamp is not restricted to the UV power bands, and there is a substantial output in the IR band as well as the visible light band. The output in the IR band causes increased heat generation, which can deform heat-sensitive articles, and/or degrade or over-cure the coating.
UV fluorescent bulbs do not have substantial IR output, and generate significantly less heat than arc lamps. However, utilization of UV fluorescent bulbs has been minimally adopted or accepted by the industry due to the low power output of such bulbs and the belief that such bulbs cannot provide sufficient UV energy to efficiently and effectively cure UV curable coating on elongate articles.
There remains a need for a UV curing system that can be efficiently and easily scaled-up, while maintaining consistent exposure of the UV spectrum and energy seen at each article of a plurality of articles, minimizing heat applied to each article, and utilizing short dwell times to increase throughput and to improve the lifespan of the UV source.